This invention relates to an image forming apparatus such as a full-color copying machine or a color printer.
As a type of fixing device, there are heating/pressurizing type fixing devices having a fixing belt system that are used for copying machines and printers adopting an electro-photography system.
Such a fixing device comprises an endless fixing belt wound around a fixing roller and a heating roller so as to make the fixing roller to be pressed against a pressurizing roller by way of the fixing belt.
A heater lamp is arranged in the inside of the heating roller and, and as the heating section of the fixing belt that is heated by the heating roller gets to the nip section comprising the fixing roller and pressurizing roller so as to contact with a sheet of recording paper that passes through the nip section, the unfixed image on the sheet of recording paper is fixed under the effect of heat and pressure.
Such color copying machines and color printers are adapted to use recording paper that may be thin paper, ordinary plain paper or cardboard of a variety of types.
In such a color copying machine or a color printer, the operation of driving the fixing device and the recording paper conveying system is stopped immediately after the completion of a printing sequence. Then, the temperature of the heating roller and the fixing belt is raised by the residual heat. Therefore, the temperature of the fixing belt rises above the target level when a printing operation is conducted in a plain paper mode immediately after a printing operation in a cardboard mode to consequently give rise to problems such as high temperature offset (where some of the toner fixed to the sheet comes off).
In an attempt to avoid this problem, an image forming apparatus is so controlled that no sheets of recording paper pass through it until the temperature of the fixing belt reaches the target level. Then, however, there arises a problem that a printing operation in a fast copy mode is a time consuming one because the cooling rate is low when the fixing belt is left to cool down spontaneously.
The object of this invention is to provide an image forming apparatus comprising a fixing device having a fixing belt that can reduce the standby time for an operation of fixing an image on a sheet of plain paper or thin paper when the operation is conducted immediately after fixing an image on a sheet of cardboard.
According to the invention, the above object is achieved by providing an image forming apparatus comprising a fixing device for fixing a color developer image on an image forming medium having a thickness selected from a number of different thicknesses, the fixing device including:
a fixing roller for fixing a color developer image of an image forming medium having a thickness selected from a number of different thicknesses at temperature selected from a number of different temperatures corresponding to the thicknesses of image forming medium;
a heating roller containing a heat source and adapted to be heated by the heat source;
a fixing belt wound around the fixing roller and the heating roller and adapted to move around the rollers so as to transmit heat from the heating roller to the fixing roller;
a first drive section for driving the heat source;
a second drive section for rotating the fixing roller;
a detector for detecting the temperature of the heating roller;
a first control section for controlling the operation of the first drive section of driving the heat source and that of the second drive section of rotating the fixing roller to rotate according to the temperature detected by the detector; and
a second control section adapted to stop the operation of driving the heat source in a rotating state of the fixing roller after the passage of a thick image forming medium through the nip section of the fixing roller for the purpose of fixing an image on an image forming medium having an ordinary thickness or a thickness smaller than the ordinary thickness to immediately succeed the operation of fixing an image on the thick image forming medium and stop the operation of rotating the fixing roller upon detecting a predetermined temperature by means of the detector.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.